Production of ethyl chlorothioformate

ABSTRACT

An improved process for producing alkyl, lower cycloalkyl, lower cycloalkylmethyl, lower alkenyl, phenyl, benzyl, chloro-substituted phenyl and certain haloalkyl chlorothioformates by the reaction of the corresponding mercaptan with phosgene. The process is conducted in two stages, both occurring in a continuous liquid phase, in the presence of an activated carbon catalyst. Production of by-product disulfide is minimized and overall capacity can be increased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of my earlier application, Ser. No.636,266, filed Nov. 28, 1975, now U.S. Pat. No. 4,012,405.

BACKGROUND AND PRIOR ART

This invention relates to the production of chlorothioformates by thereaction of a mercaptan with phosgene in the presence of an activatedcarbon catalyst, ##STR1## In this invention, R is alkyl, lowercycloalkyl-methyl, lower cycloalkyl, lower alkenyl, phenyl,chloro-substituted phenyl, benzyl or chloro-substituted alkyl in whichthe chloro substituent is situated at least as far as the γ (gamma)carbon atom, with respect to the sulfur atom. By the term "alkyl" or"chloro-substituted alkyl" is meant such groups having from 1 to 15,preferably from 1 to 10, and most preferably from 1 to 6, carbon atoms,for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec.- butyl,isobytyl, n-pentyl, neopentyl, n-hexyl, neohexyl, n-heptyl, n-octyl,n-decyl, n-dodecyl and n-tetradecyl. By "lower alkenyl" is meant suchgroups having from 2 to 5 carbon atoms and at least one olefinic bond.By "lower cycloalkyl" is meant cycloaliphatic groups having from 3 to 7carbon atoms, such as cyclopropyl and cyclohexyl. The term "lowercycloalkyl-methyl" includes groups having from 3 to 7 carbon atoms inthe cyclo alkyl portion, such as cyclopropylmethyl andcyclopentylmethyl. The term "chlorophenyl" includes both mono- andpolychlorinated phenyl rings in which the chlorine atom or atoms may bevariously substituted.

In a preferred embodiment of this process, R is alkyl, lower cycloalkyl,lower cycloalkyl-methyl, benzyl, phenyl or chloro-substituted phenyl.Preferred embodiments for the various possibilities for R are: foralkyl- such groups having from 1 to 6 carbon atoms, particularly ethyl,n-propyl, isopropyl, n-butyl, sec.-butyl, n-pentyl and neopentyl; forlower cycloalkylcyclobutyl; for lowercycloalkyl-methyl-cyclopropylmethyl and cyclopentylmethyl; for the loweralkenyl- allyl; for chloro-substituted-phenyl-, p-chlorophenyl; for thehaloalkyls- 3-chloropropyl.

Such chlorothioformates are useful intermedites for the production ofherbicidally effective thiocarbamates and similar compounds. Thisreaction between mercaptans and phosgene to produce chlorothioformatesis described in U.S. Pat. No. 3,165,544 of Harry Tilles, which disclosesthe conduct of this process in laboratory size equipment. It is pointedout that reaction temperatures should be maintained as low as possible,consonant with reasonable reaction rates since at high temperatures adisulfide by-product begins to form in significant amounts. Maximumtemperatures are suggested for this reaction of between about 70° and140° C.

One process which has been utilized for commercial scale production oflower alkyl chlorothioformates by this reaction employs two catalyticbeds of activated carbon arranged in series. The first bed is preferablycontained in tubes of a multi-tube reactor; the second is in the form ofa packed bed reactor containing a single catalyst bed. The first reactoris operated as a continuous liquid phase reactor; more specifically asan upflow tubular catalytic reactor, with starting materials introducedat the bottom and products removed from the upper portion. The partiallyreacted mixture is then introduced into the top of the second reactor,which functions as a trickle-flow (downflow) packed bed. That is, thesecond reactor is operated in the continuous gas phase since gaseoushydrogen chloride product is continuously passing through the bed.Reaction products are removed from the lower portion of the secondreactor and passed to downstream apparatus for separating thechlorothioformate. Operation of this process for production of ethylchlorothioformate, however, has been found to produce this product in apurity of only between about 91 and about 95%. The major impurity isdiethyl disulfide, present in about 3-7% concentration, with most of theremaining impurities being diethyl dithiocarbonate. When used to producen-propyl chlorothioformate, the amount of disulfide by-product rangedfrom 1.5-13.7% and averaged just under 5% and the chlorothioformatepurity averaged about 93%.

It is an object of the present invention to provide an improved processfor the production of chlorothioformates by reaction of a mercaptan andphosgene in the presence of an activated carbon catalyst.

A further objective of the present invention is to provide such aprocess with minimization of a disulfide by-product.

A third objective of the present invention is to provide such a processwith enhanced production capacity.

Yet another object of the present invention is to provide such a processhaving good temperature control in the reactors.

A still further objective of the present invention is to provide such aprocess having a good conversion of the mercaptan to chlorothioformate.

SUMMARY OF THE INVENTION

The present invention comprises a process for production ofchlorothioformates having the formula ##STR2## , in which R is alkyl,lower cycloalkyl, lower cycloalkyl-methyl, lower alkenyl, phenyl,chloro-substituted phenyl, benzyl or chloro-substituted alkyl in whichthe chloro substituent is situated at least as as far as the gammacarbon atom, with respect to the sulfur atom, by the reaction of thecorresponding mercaptan with phosgene in the presence of an activatedcarbon catalyst comprising: (a) contacting the mercaptan with phosgenein a first continuous liquid phase reaction zone in the presence of acatalyst comprising activated carbon; (b) removing a first reactionproduct from the first reaction zone; (c) contacting the first reactionproduct with a catalyst comprising activated carbon in a secondcontinuous liquid phase reaction zone; and (d) removing a secondreaction product comprising the chlorothioformate from the secondreaction zone.

DETAILED DESCRIPTION OF THE INVENTION

The invention is more particularly described with reference to theFIGURE, which shows a generalized flow sheet for the conduct of theprocess.

Referring to the FIGURE, a mercaptan in line 1 is combined with phosgenein line 2 and the mixture introduced through line 4 into the lowerportion of a first reactor 10. Reactor 10 is operated with reactants andproducts in a continuous liquid phase. Preferably, reactor 10 is atubular packed bed reactor containing a plurality of tubes filled withactivated carbon of an appropriate particle size such that each tubefunctions in the conventional manner as a miniature packed bed reactor.The reactants in stream 4 are introduced into the lower portion of thereactor, thereby into the lower portions of the individual tubes, andpass upwards through the tubes. The average outlet temperature isgenerally between about 0° and about 70° C. preferably between about 0°and about 50° C. Pressures range between about 0 and about 150 psig,preferably between about 0 and about 50 psig.

The partially reacted products from the first reactor 10 are removedfrom the upper part of this reactor as overhead in line 6 and passedthrough line 8 into a second reactor 11. If desired, gaseous productsfrom reactor 10 may be separated from the mixture in line 6 prior to itsintroduction into reactor 11. Reactor 11 contains a packed bed 12 ofactivated carbon. The reaction is complete in reactor 11 in a continuousliquid phase. As shown in the FIGURE, this is accomplished byintroducing reactants into the lower portion of reactor 11 so that thisreactor operates in so- called "flooded upflow" condition. The reactoris generally operated at average outlet temperatures of between about 0°and about 70° C., preferably between about 10° and about 50° C., mostpreferably at a temperature within this range below 50° C. Pressuresrange between about 0 and about 150 psig, preferably between about 0 andabout 50 psig. Residence time of the reactants in reactor 11 isgenerally between about 1 and about 180 minutes, preferably betweenabout 5 and about 90 minutes.

The reaction products are removed from reactor 11 though overhead line9, passed to separation drum 13 and product chlorothioformate is removedin line 15 for further purification. Gaseous by-products (primarilyhydrogen chloride with some unreacted phosgene) are taken off at line 14and passed to downstream purification units (not shown) for recovery ofunreacted starting materials for recycle and removal and furtherprocessing of hydrogen chloride.

When, as in the prior process, the second reactor 11 is operated as acontinuous gas phase reactor (e.g. as a trickleflow packed bed reactor)the average outlet temperature can also be maintained at between about0° C. and about 70° C., as in the present process. However, operationaccording to the prior process results in an uneven temperature profileacross the reactor due to poor heat transfer, providing localized hightemperature zones, or "hot spots". It is known, from U.S. Pat. No.3,165,544, that undesirably high temperatures contribute to theformation of by-product disulfide. The presence of hot spots in reactor11, therefore, increases the possibility of formation of this byproduct.

When the process is practiced using the present invention, however theoperation of the second reactor 11 as a continuous liquid phase packedbed reactor results in a marked decrease in disulfide formation sincesuch operation provides better heat transfer and a more uniformtemperature distribution throughout the catalyst bed.

Operation according to the present invention, with reactor 11 acontinuous liquid phase reactor, results in an increase in the residencetime in the second reactor at the same flow rate as the previousprocess, by a factor of at least about 10. In the previous process, forinstance, residence time in this reactor was often in the order of 4-5minutes. In the present process the residence time may be between about5 and about 180 minutes, or even longer, according to the flow rate.Preferably, the residence time is between about 45 and about 180minutes, more preferably between about 45 and about 90 or 120 minutes.It could reasonably be expected that operation as such longer residencetimes could result in increased by-product formation; however, it wasfound, surprisingly, that operation at such long residence times doesnot result in increased by-product formation so long as the temperatureis maintained under good control. Alternatively, the flow rate ofmaterials can be increased to permit operation at lower residence timesin this reactor and increased capacity, as well as an increasedconversion of mercaptan to chlorothioformate. Preferably the flow ratecan be increased up to 2-21/2 times that used previously. At increasedflow rates, residence time in the first reactor 10 is also decreased.

The desired temperature control in reactor 11 and in the overall processcan be augmented by introduction of excess liquid phosgene into thesystem, either as part of the feed in line 2 or separately, into thereactor 10. Part or all of this excess will vaporize under the normaloperating conditions of reactor 11, the vaporization absorbing heatgenerated during the reaction.

As an alternative method of temperature control, and also to assist inincreasing the overall production of chlorothioformate, a relativelycold recycle stream 5, obtained from downstream processing units (notshown), and comprising primarily unreacted starting materials, can beintroduced into the system. The recycle stream in line 5 can beintroduced into reactor 11 via lines 7 and 8; its presence contributesto the maintenance of a desirably low temperature in reactor 11,preferably one below about 50° C. Alternatively, recycle stream 5 can beintroduced via lines 3 and 4 into the first reactor 10. Most preferably,temperature control is maintained by a combination of utilization ofexcess liquid phosgene and introduction of the recycle stream intoreactor 11.

Operation according to the invention, as will be further seen from theexamples which follow, results in conversion of approximately 94% ofstarting ethyl mercaptan and production of a product of about 98%purity, containing generally less than 1% diethyl disulfide.Additionally, the use of a continuous liquid phase reactor, through theincrease in residence time, provides greater capacity than a similarunit operating using a downflow or trickle flow packed reactor, in whichthe residence time is substantially shorter. Similar results were foundin the case of n-propyl chlorothioformate, as can be seen from Example3. On the basis of these results and the general knowledge in this art,for example the information contained in U.S. Pat. No. 3,165,544, it isreasonable to expect similar good performance for the other types ofcompounds included herein. As an alternative to the "flooded upflow"type of reactor shown in the FIGURE, reactor 11 can be operated as acontinuous liquid phase reactor in any other manner as may beconvenient, for example as a downflow flooded packed bed reactor.

The following examples illustrate the practice of the present invention.

EXAMPLE 1

A two-reactor system is utilized as shown in the FIGURE, having acapacity for production of about 57,000 pounds per day of ethylchlorothioformate. The first reactor is a tubular upflow reactor, withthe tubes packed with activated carbon catalyst. The second reactor is apacked bed reactor containing a bed of carbon catalyst and is operatedas an upflow reactor.

Into the first reactor, corresponding to reactor 10 of the FIGURE, arefed 22.4 lb.-moles/hr. of phosgene and 20.4 lb.-moles/hr. of ethylmercaptan. The reactor is operated at an inlet temperature of about15°-40° C., an outlet temperature of about 50°-65° C., and an outletpressure of about 30-36 psig. The partially reacted products from thefirst reactor are fed into the lower portion of the second reactortogether with a recycle stream containing 10.7 lb.-moles/hr. phosgeneand 4.7 lb.-moles/hr. ethyl chlorothioformate. The second reactor isoperated at an inlet temperature of about 18°-26° C., an outlettemperature of about 33'-49° C., an outlet pressure of about 24-28 psig,and a residence time of about 75 minutes.

Conversion of ethyl mercaptan to the chlorothioformate was 94%. Theproduct was produced in 98% purity, containing about 0.5-1% diethyldisulfide and about 1% diethyl dithiocarbonate.

EXAMPLE 2

The same system was utilized as in Example 1, but flow rates ofmaterials were increased to provide a capacity of about 114,000 lbs./dayof ethyl chlorothioformate. The flow rates of feed phosgene and ethylmercaptan were respectively 44.8 and 40.8 lb.-moles/hr. The recycle flowrate was 21.4 and 9.4 lb.-moles/hr. respectively of phosgene and ethylchlorothioformate. Operating temperatures and pressures weresubstantially the same as in Example 1. The residence time of materialsin the second reactor was reduced to about 35 minutes. The product ethylchlorothioformate was again obtained in 98% purity, with 94% conversionof ethyl mercaptan. Diethyl disulfide content of the product was about0.5-1%; diethyl dithiocarbonate content was about 0.5%.

EXAMPLE 3

A two-reactor system is utilized as shown in the FIGURE, having acapacity for production of about 74,000 pounds per day of n-propylchlorothioformate. The first reactor is a tubular upflow reactor, withthe tubes packed with activated carbon catalyst. The second reactor is apacked bed reactor containing a bed of carbon catalyst and is operatedas an upflow reactor.

Into the first reactor, corresponding to reactor 10 of the FIGURE, arefed 24.6 lb.-moles/hr. of phosgene and 22.4 lb./moles/hr. of n-propylmercaptan. A recycle stream containing about 11 lb.-moles/hr. phosgeneand about 5 lb./moles/hr. n-propyl chlorothioformate is also introducedinto reactor 10. The reactor is operated at an inlet temperature ofabout 15°-40° C., an outlet temperature of about 40°-55° C., and anoutlet pressure of about 26-30 psig. The partially reacted products fromthe first reactor are fed into the lower portion of the second reactor.The second reactor is operated at an inlet temperature of about 40°-55°C., an outlet temperature of about 40°-55° C., an outlet pressure ofabout 22-26 psig, and a residence time of about 75 minutes.

Conversion of n-propyl mercaptan to the chlorothioformate was 94%. Theproduct was produced in 98-99% purity.

What is claimed is:
 1. A process for production of a chlorothioformatehaving the formula ##STR3## in which R is C₁ -C₁₅ alkyl,cycloalkylmethyl having from 3 to 7 carbon atoms in the cycloalkylportion, C₃ -C₇ cycloalkyl, C₂ -C₅ alkenyl, phenyl, chloro-substitutedphenyl, benzyl or chloro-substituted C₁ -C₁₅ alkyl in which thechloro-substituent is situated at least as far as the gamma-carbon atom,with respect to the sulfur atom, comprising:(a) contacting a mercaptanhaving the formula RSH with phosgene in a first continuous liquid phasereaction zone in the presence of a catalyst comprising activated carbon;(b) removing a first reaction product from the first reaction zone; (c)contacting the first reaction product with a catalyst comprisingactivated carbon in a second continuous liquid phase reaction zone; and(d) removing a second reaction product comprising the chlorothioformatefrom the second reaction zone.
 2. A process according to claim 1 inwhich R is alkyl.
 3. A process according to claim 2 in which R is alkylhaving from 1 to 10 carbon atoms.
 4. A process according to claim 2 inwhich R is alkyl having from 1 to 6 carbon atoms.
 5. A process accordingto claim 4 in which R is n-propyl.
 6. A process according to claim 1 inwhich R is cycloalkyl.
 7. A process according to claim 6 in which R iscyclohexyl.
 8. A process according to claim 1 in which R is benzyl.
 9. Aprocess according to claim 1 in which R is phenyl.
 10. A processaccording to claim 1 in which R is chlorosubstituted phenyl.
 11. Aprocess according to claim 10 in which R is p-chlorophenyl.
 12. Aprocess according to claim 1 in which step (c) is operated at an averageoutlet temperature of between about 0° and about 70° C.
 13. A processaccording to claim 1 in which step (c) is operated at an average outlettemperature of between about 10° and about 50° C.
 14. A processaccording to claim 1 in which step (c) is operated at an average outlettemperature of between about 10° and below about 50° C.
 15. A processaccording to claim 1 in which step (c) is operated at a residence timeof between about 5 and about 180 minutes.
 16. A process according toclaim 15 in which step (c) is operated at a residence time of betweenabout 45 and about 180 minutes.
 17. A process according to claim 1 inwhich an excess of liquid phosgene is introduced into step (a).
 18. Aprocess according to claim 1 in which an excess of liquid phosgene isintroduced into step (c).
 19. A process according to claim 1 furthercomprising recovering unreacted starting materials from the product ofstep (d) and recycling said unreacted starting materials to step (c).20. A process according to claim 1 further comprising recoveringunreacted starting materials from the product of step (d) and recyclingsaid unreacted starting materials to step (a).
 21. A process accordingto claim 1 further comprising recovering the chlorothioformate from theproducts of step (d).
 22. A process according to claim 1 in which step(c) is conducted by introducing the first reaction product into thelower portion of a packed bed reactor containing a bed of activatedcarbon catalyst.
 23. In a process for the production of an alkylchlorothioformate by the reaction of an alkyl mercaptan with phosgene inthe presence of a catalyst comprising activated carbon in a systemcomprising two reactors operating in a series, the improvementcomprising operating the second reactor as a continuous liquid phasereactor.
 24. A process according to claim 23 in which the second reactoris operated as a flooded upflow packed bed reactor.
 25. A processaccording to claim 23 in which the alkyl mercaptan and the alkylchlorothioformate have from 1 to 6 carbon atoms in the alkyl group. 26.A process according to claim 25 in which the alkyl group in n-propyl.